One conventional technique for attaching a lead to a semiconductor chip involves the use of a tin/lead solder, the typical 60/40 tin/lead solder forming a eutectic having a melting point of 182.degree.C. Conventional fluxing agents, such as rosin flux, which leave a surface film which must be removed (for example, by rubbing with alcohol) before soldering or which contain ingredients deleterious to the semiconductor chip, are obviously to be avoided. As described in U.S. Pat. No. 3,665,590, hot hydrogen reducing gas may be used to combine with the tin oxide and lead oxide of a surface oxidized tin/lead solder to form water vapor and melted and purified tin/lead solder. However, the formation of water vapor, formed either through reaction of the hydrogen with the oxide impurities or through burning of the hydrogen, adjacent to solder surface is clearly undesirable.
In order to obtain solder bonds of great strength and thus more capable of resisting the rough treatment to which the semiconductor devices are often subjected, it has been proposed to use a lead solder which forms a bond of greater physical strength than a tin/lead solder. A typical lead solder may consist exclusively of lead, or include a small amount of calcium (typically about 0.08% by weight), antimony (typically about 8-10% by weight) or other metals for additional strength. As pure lead has a melting point of 327.degree.C, the lead solder bonds are not only stronger, but capable of withstanding higher temperatures than conventional tin/lead solders. A further advantage of the lead solder is that it is a relatively soft material capable of accommodating the differences in thermal expansion coefficients between typical semiconductor materials, such as silicon, and typical lead materials, such as silver and aluminum. On the other hand, the absence of tin from the solder aggravates to a substantial degree the problem caused by the presence of lead oxide on the surface of the lead solder. Indeed, removal of lead oxide from a lead solder surface with hydrogen gas would require temperatures in the range of 800.degree.C.
Accordingly, it is an object of the present invention to provide a process for removing lead oxide from the surface of the lead solder of a semiconductor assembly in the inert atmosphere of a joining furnace by a process which does not require manual removal of the flux or its reaction products from the solder surface.
It is another object to provide such a process which operates at temperatures well below 800.degree.C.